Anal. Chem. 2002, 74, 648-654
Hollow Fiber-Protected Liquid-Phase Microextraction of Triazine Herbicides Gang Shen and Hian Kee Lee*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Republic of Singapore 117543
A new microextraction technique termed hollow fiberprotected liquid-phase microextraction (LPME) was developed. Triazines were employed as model compounds to assess the extraction procedure and were determined by gas chromatography/mass spectrometry. Toluene functioned as both the extraction solvent and the impregnation solvent. Some important extraction parameters, such as effect of salt, agitation, pH, and exposure time were optimized. The new method provided good average enrichment factors of >150 for eight analytes, good repeatability (RSDs 3.00. They were completely protonated at pH 2.00, which made them partition much more into the aqueous phase. Thus, in acidic solution, their extraction efficiencies were very low and could not be extracted into the organic phase. At pH >4.00, they could exist as neutral molecules and were easily extracted. As far as the remaining three trazines (simazine, atrazine, propazine) were concerned, since their pKa values were ∼2.00, the extraction efficiency did not change significantly when pH was 2.00. In general then, a pH of 4.0 or greater was conducive for extraction of all triazines. For convenience, therefore, no adjustment of pH was made for subsequent experiments since neutral pH conditions were suitable for extraction. This conclusion is consistent with that normally applicable to SPME in the extraction of triazines. Exposure Time. A series of exposure times was investigated by extracting spiked solutions (containing 20 µg/L of each analyte) at 1000 rpm agitation. For all target analytes, the amount extracted increased dramatically with increasing exposure time from 1 to 20 min (Figure 6). After 20 min, the curves became flat and the
enrichment factor increased only slightly. The extraction kinetics are similar to those generally observed for SPME, which normally takes considerable time before reaching equilibrium. It is therefore undesirable to use an extraction method based on equilibrium time. Additionally, it is desirable that the extraction time be shorter than the chromatographic running time (in the present experiments, this was 27.8 min) in order to obtain a reasonable sample throughput. Thus, from a practical point of view, 20 min was used in this study. Based on the above results, optimized hollow fiber-protected LPME conditions for triazines were as follows: 3 µL of toluene to extract 3 mL of sample; 20-min extraction time with neutral pH; stirring at 1000 rpm and 10% of sodium chloride. Humic Acids. Further experiments were conducted that focused on the effect of humic acids on hollow fiber-protected LPME. The concentration of humic acids was varied in the range of 0-200 mg/L levels. In Figure 7, it is indicated that the addition of humic acids did not significantly decrease the compounds extracted when a 1 mg/L amount was introduced. The extraction efficiencies were constant as the concentrations of humic acids
652 Analytical Chemistry, Vol. 74, No. 3, February 1, 2002
Table 2. Enrichment Factors of Hollow Fiber-Protected LPME and Static Solvent Drop LPME enrichment factors compounds
hollow fiberprotected LPME
static solvent drop LPME
simazine atrazine propazine secbumeton sebuthylazine desmetryn simetryn prometryn
42 141 178 165 190 170 179 208
19 38 41 40 41 43 43 42
added were varied from 1 to 200 mg/L. In SPME,8 the extraction efficiency decreased dramatically when the humic acids in solution were over 100 mg/L. The reason for the observation probably lies in the selectivity of the hollow fiber membrane. The micropores of the membrane wall allow the low molecular weight target analytes to diffuse through while excluding high molecular weight interfering compounds. Humic acids typically have molecular masses up to several million daltons and thus cannot be extracted into the organic solvent. For SPME, the fiber is in direct contact with the humic acids, which compromises the extraction. Enrichment Factors. The optimal conditions were employed to investigate enrichment factors. In this case, static solvent drop LPME was also carried out in order to compare these two methods. As shown in Table 2, use of the hollow fiber membrane to protect the solvent significantly improves the extraction process. The enrichment factors of static LPME were from 19 to 42, in comparison to those of the new method, which were from 42 to 208 (average 159). Method Validation. Linearity, Limits of Detection, and Repeatability. To investigate the linearity of hollow fiber-protected LPME, 0.5-50 µg/L solutions of the herbicides were prepared in deionized water. All triazines exhibited good linearity with squared regression coefficients (r2) > 0.9995 (Table 3). This allowed the quantification of these compounds by the method of external standardization. Limits of detection (LODs) of triazines studied in the aqueous sample, calculated on the ratio of signal to noise at 3 (S/N ) 3) under MS-SIM conditions, were in the range 0.007-0.063 µg/L. The repeatability study was performed by extracting aqueous sample spiked at 5 µg/L of each compound (seven replicates). The relative standard deviations (RSDs) were calculated to be from 0.92 to 3.43%. The good repeatability can be explained in two ways. One is the manually cut hollow fiber had no significant effect on the precision. Another is that the protection offered by the hollow fiber made the solvent drop stable, and the effect of the matrix on the extraction solvent was eliminated. Recoveries and Precision. The proposed microextraction procedure was used for determination of triazines in Nanopure water at spiked concentration levels of 2, 10, and 20 µg/L. The relative recoveries and precision (three replicates) are listed in Table 3. As can be seen, the recoveries were in the range 90.5-111.9% and RSDs were from 0.78 to 3.84%. Extraction of Triazines from Slurry. In view of the protection, which gave obvious benefits as compared to unprotected
static LPME, afforded to the solvent by the hollow fiber membrane, we applied the technique to extract the triazines in a complex matrix, using the previously determined optimum extraction conditions. Slurry samples (20 mg of soil/mL of water) were employed, while SPME was also performed as comparison in terms of precision and LODs. Static LPME was also attempted, which confirmed that the solvent drop could not be stably maintained at the syringe needle tip because of the effect of the particles in the slurry sample. No further experiment was carried out. As seen from Table 4, SPME gave poorer precision, with RSDs ranging from 3.90 to 16.1% for 30-min extractions. In comparison, the precision of hollow fiber-protected LPME was 0.9995 vs r2 >0.9974) and repeatability (0.92-3.43 vs 2.33-4.87%) were achieved as well. Compared with EPA method 507 (LODs from 0.1 to 0.2 µg/L), the newly developed microextraction procedure can achieve LODs ranging from 0.007 to 0.063 µg/L, exceeding the requirement for triazine analysis in aqueous samples. In addition, because of the selectivity of the porous hollow fiber membrane, the technique (20) Beltran, J.; Lopez, F. J.; Hernandez, F. J. Chromatogr., A 2000, 885, 389404. (21) Barshick, C. M.; Barshick, S. A.; Britt, P. F.; Lake, D. A.; Vance, M. A.; Walsh, E. B. Int. J. Mass Spectrom. 1998, 178, 31-41. (22) Zhang, Z.; Poerschmann, J.; Pawliszyn, J. Anal. Commun. 1996, 33, 219221. (23) Handley, A. J., Ed. Extraction Methods in Organic Analysis; Sheffield Academic Press: Boca Raton, FL, 1999; p80.
Analytical Chemistry, Vol. 74, No. 3, February 1, 2002
653
Table 3. Relative Recoveries, Precision (RSDs, n ) 3), Linearity, and LODs (S/N ) 3) of Hollow Fiber-Protected LPME spiked deionized water samples (%) compounds
2 µg/L
10 µg/L
20 µg/L
linearity r2
LODs (µg/L)
simazine atrazine propazine secbumeton sebuthylazine desmetryn Simetryn Prometryn
111.9 ( 2.72 100.7 ( 3.02 101.3 ( 3.71 97.7 ( 1.78 104.5 ( 0.78 101.8 ( 1.59 102.1 ( 1.86% 105.7 ( 1.11%
98.1 ( 3.17 96.1 ( 2.94 95.9 ( 1.03 92.1 ( 3.76 94.3 ( 2.02 95.0 ( 2.68 92.5 ( 2.98% 93.9 ( 2.18%
93.5 ( 3.84 94.8 ( 2.60 95.9 ( 1.10 90.5 ( 4.38 94.3 ( 2.68 91.8 ( 2.51 92.9 ( 3.01% 94.0 ( 2.39%
0.9996 0.9999 0.9997 0.9995 0.9997 0.9997 0.9995 0.9996
0.063 0.014 0.010 0.021 0.010 0.009 0.012 0.007
Table 4. Extraction of Triazines from Slurry Samplea by Hollow Fiber-Protected LPME and SPME (n ) 3) hollow fiber-protected LPME (20-min extraction)
SPME (30-min extraction)
compounds
% recovery
% RSDs
LODb (ng/g)
% recovery
% RSDs
LODb (ng/g)
simazine atrazine propazine secbumeton sebuthylazine desmetryn simetryn prometryn
88.8 96.8 98.9 89.0 97.5 93.0 94.6 99.8
4.99 3.78 3.61 4.17 3.87 4.20 3.81 3.51
0.18 0.10 0.08 0.13 0.07 0.07 0.09 0.04
95.9 94.8 94.4 85.0 93.5 90.9 91.6 99.0
7.19 8.42 4.96 16.1 12.8 3.90 6.39 10.3
0.15 0.05 0.06 0.04 0.02 0.03 0.03 0.02
a
Slurry sample spiked at 5 µg/L for each compound. b Calculated from 1 µg/L spiked level, S/N ) 3
could be used to extract triazines from “dirty” matrixes. The comparison between the present technique and SPME indicates that hollow fiber-protected LPME is more precise than SPME in extracting triazines from slurry samples, although the latter is able to provide better LODs possibly due to the slightly longer extraction time. On the basis of these considerations, hollow fiberprotected LPME is not only a good sample preconcentration technique but also an excellent sample cleanup procedure, which makes it directly applicable to dirty samples. Hollow fiberprotected LPME is also conveniently compatible with GC, but unlike LLE, there is no need for evaporation of extract before injection. For every new extraction, a fresh piece of fiber (only 1.3-cm length was used) was employed. Thus the possibility of carry-over was eliminated. Compared with commercially available SPME fibers (each costs ∼$70 U.S.), polypropylene hollow fiber is considerably less costly (one bundle of 2600 pieces with 53.5-
654 Analytical Chemistry, Vol. 74, No. 3, February 1, 2002
cm length costs only ∼$200 U.S.). Finally, the advantages of hollow fiber-protected LPME allows its potential application as a sample preparation and cleanup technique for drug analysis from biological matrixes. This is currently being investigated. ACKNOWLEDGMENT The authors are grateful to the National University of Singapore for financial support of this work. G.S. thanks the university for the award of a research scholarship. Ms. C.N. Tang is gratefully acknowledged for providing technical assistance. Received for review May 17, 2001. Accepted October 18, 2001. AC010561O
